This invention relates to a method of lost foam casting and, in particular, it relates to a method of lost foam casting wherein the pattern is first reduced in size and then removed from the mold prior to pouring.
The lost wax precision casting technique is an extremely favorable molding process from many standpoints, in particular the high level of the as-cast quality of the product of lost wax molding. High as-cast quality reduces machining and manufacturing costs, allows closer cast tolerances, and overall increases productivity. Unfortunately, the lost wax process is impractical and uneconomical to use for most products in conventional steel foundries, and is limited mainly to the manufacture of relatively small components, because the wax pattern tends to shrink and distort more as the size increases. The extreme weight of larger wax patterns can also cause handling problems.
A newer process which solves some of the problems of the lost wax process was developed by the Steel Castings Research and Trade Association (SCRATA) of Great Britain and is termed the Replicast Ceramic Shell Process. This process includes the use of expanded polystyrene (EPS) in place of the wax used to form the pattern. According to SCRATA, the use of EPS offers several advantages over wax. According to SCRATA, since EPS expands less on heating, pattern removal is facilitated and the shell may be thinner and still avoid cracking. It is true that thinner ceramic shells are lighter, less expensive and easier to handle. However, applicants have found that the EPS pattern contains air and other gases which, when heated, often expand even more than wax had, and cracking of the ceramic shell can be aggravated by employing conventional mold construction methods with an EPS pattern.
This invention relates to improvments over the methods described above and to solutions to the problems raised thereby.